It is known in automotive technology to install a permanent-magnet-excited synchronous machine (PM synchronous machine) as an integrated crankshaft starter-generator into the power train of a vehicle between the internal combustion engine and the transmission.
To prevent the permanent demagnetization of the permanent magnets during operation of the synchronous machine, the maximum stator field at an elevated rotor temperature must be reduced in terms of a temperature-dependent “derating” compared to the stator field at low temperatures. In particular in stop-start operation of the integrated crankshaft starter-generator it is desirable that the regulated stator current is as close as possible to a predetermined derating curve in order to ensure high starting torque and thus a short starting time of the internal combustion engine. To achieve this object, it is necessary to know the rotor temperature as accurately as possible.
One possibility for obtaining information about the rotor temperature is to measure the temperature in the rotor itself by using a thermometer. However, due to cost considerations, this possibility is rather unattractive in automotive technology.
To measure the coolant temperature and the temperature in the power converter and to draw conclusions about the rotor temperature from the measured temperature values by using an approximation algorithm is another possibility. This method is comparatively complicated and yields in addition only inaccurate results. Furthermore, this method is dependent on the respective application since the heat models are different in almost each application.
A method of regulating a synchronous three-phase machine using a converter bridge is described in German Published Patent Application No. 100 44 181. In this method, the field current flowing through the field winding is regulated in such a way that the output voltage of the synchronous three-phase machine reaches a setpoint value and the currents of the phases of the synchronous three-phase machine are regulatable in at least two control ranges. Additionally in this method, actual input variables id and iq, transformed from the three-phase system R-S-T, are supplied to a correction device from whose output variables setpoint input variables udsetpoint and uqsetpoint are determined. A multi-variable regulation of a claw-pole three-phase machine is implementable by using the controller structure proposed in German Published Patent Application No. 100 44 181.
A device having means for determining temperatures for regulating a generator is known from European Published Patent Application No. 0 462 503. The advantage of this known device lies in the fact that, in the event of high power demand, the generator may be operated in an overexcited range, and permanent damage to the generator, the rectifier diodes, or the voltage regulators as a result of thermal overheating is prevented due to the temperature determination which takes place by measuring the temperature in the voltage regulator itself and due to safeguards which may be appropriately applied. The safeguards include that prior to or when reaching a predetermined temperature limit, the field current is reduced. The means for determining the temperature include a microcomputer in which required characteristics are stored, to which the necessary measured variables are supplied, and in which the necessary computations are executed.